The present invention relates to the production of curved and tempered (toughened) glass, especially thin sheets.
Such products are frequently used as windows for vehicles in general, more especially for automobiles.
This application requires that the windows be accurately made, both as regards the required dimensions and tolerances of curvature, as well as the configuration to correspond to the general shape of the vehicle body.
Moreover, it is necessary that the visibility through such windows should not be impaired by optical distortions or by flaws in the glass.
As is well known, tempered glass sheets, especially those used in the automobile industry, can also be manufactured by horizontal machines which permit high outputs.
In these machines, each glass sheet is conveyed horizontally by a roller conveyor during the consecutive heating, forming (if required) and tempering phases.
In the case of curved glass sheets, these sheets are collected at the output of a forming press which forms them into the required shape.
After the forming operation, the piece leaves the forming press and passes on to the tempering zone via a roller conveyor.
In order for a piece to be efficiently tempered, it must still retain a certain degree of plasticity.
This plasticity, which is an essential condition for tempering, inevitably leads to deformation of the shape previously assumed by the glass when submitted to treatment in the forming press.
Causes of deformation are essentially as follows:
(a) the glass's own weight
(b) inertia of the glass sheet
(c) action of the first blasts of hot air in the tempering zone
(d) any assymmetrical cooling between the upper and lower sides of the glass sheet.
It should be noted that the thinner is the glass sheet, the greater is the risk of deformation, since, whilst it is true that the weight and inertia decrease in proportion with the decrease in thickness of the glass sheet, it is also true that resistance to deformation decreases more or less with the cube of the thickness of the glass sheet.
The deformation due to the hot blast of air during tempering is also very marked.
During recent years, much commercial importance has been placed on thinner and thinner curved tempered glass sheets, with thicknesses passing from 5-6.5 mm down to 2.5-4 mm.
Consequently, a bending process such as that described above in which the piece travels on a roller conveyor, is particularly affected by the decrease in thickness since the resistance to deformation decreases more or less with the cube of the thickness.
The stepping up of blasts of hot air for tempering which is necessary on account of the higher temperature gradient required by the thinner glass sheets contributes, in turn, to the lower structural resistance, thereby making it difficult to control the shape of the product which, after forming, is still in the plastic state.